powerpc/asm: Use OFFSET macro in asm-offsets.c

A lot of entries in asm-offests.c look like this:

  DEFINE(TI_FLAGS, offsetof(struct thread_info, flags));

But there is a common macro, OFFSET, which makes this cleaner:

  OFFSET(TI_flags, thread_info, flags)

So use it.
Signed-off-by: Rashmica Gupta <rashmicy@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>